The objective of this study is to elucidate certain molecular features of X-chromosome inactivation. We will ask what role actual alterations of the DNA play in the inactivation process of both somatic and germ line cells. We will ask further whether inactive X DNA is functional in DNA mediated gene-transfer experiments. Preliminary results suggest that there exists a difference at the hypoxanthine phosphoribosyl transferase (HPRT) locus between active and inactive X DNA. We will attempt to isolate the mouse HPRT gene using the genetic techniques of mammalian cell DNA transformation and plasmid rescue. Such HPRT sequences will be used to analyze structurally the HPRT sequences of the inactive and active X's in both somatic and germ line cells at different developmental stages. We will also examine DNA from primary spermatocytes to study the hypothesis of a form of X inactivation during spermiogenesis.